1. Field of the Invention
The present invention relates to a loop transmission system, also called a data highway, having a plurality of stations or node processors connected in series on a looped transmission line to transfer data among the stations, in which an additional looped transmission line is provided so that one of the loop transmission lines is selectively used as an active line while the other line operates as a spare on which no data transmission occurs, and when a fault occurs in the active one of the transmission lines used for data transmission (hereinafter referred to as a current or active transmission line), the other transmission line (hereinafter referred to as a spare transmission line) is used for data transmission.
2. Description of the Prior Art
In the loop transmission system, it has been a common practice to duplicate the loop transmission line to enhance the reliability of the looped transmission path. In order to discriminate the current transmission line from the spare transmission line or switch the transmission line from the current one to the spare one, it has been proposed that a control signal line be provided in addition to the two data transmission lines to monitor the loop transmission system and control the switching of the system. In this arrangement, a control station sends a current/spare discrimination signal or a switching signal to the respective stations through the control signal line to discriminate the spare transmission line or switch the transmission line. However, since the arrangement requires the additional control signal line and a separate control circuit associated with that control signal line, the cost of the system is undesirably expensive.
When a fault has occurred in the current or active transmission line and the loop has been switched to the spare transmission line, a search for the fault location of the fault in the former current transmission line which is now the spare transmission line has been conducted, in the past, after the completion of the transmission operation, or by investigating connection units one by one, between every two terminal stations or between a supervising station and the terminal station, or by providing in each of the terminal or supervising stations at the side to be connected to the spare transmission line a circuit having the same or equivalent function as that of a circuit connected to the current transmission line to investigate the status of the line under the control of a test program.
However, all of the above methods provide a problem in that the first method of investigation after the completion of the transmission operation is not applicable to a stop-free operation system and the second method of investigation of the respective connection units needs much manpower and a long time, while the third method needs substantial hardware and is not economical.